Director valve based voltage source converters have recently been developed where there is a string of director valves connected in series between two Direct Current (DC) poles and a string of multilevel cells connected in parallel with this valve string. In this structure the director valves are used for directivity and the multilevel cells for waveshaping. A voltage source converter where the director valves are thyristors is for instance disclosed in WO2014/082657. In one embodiment of the document there are three parallel director valve phase legs and one waveshaper phase leg, where the midpoint of the waveshaper phase leg is connected to each of the midpoints via a series-circuit comprising a commutation cell and two bidirectional switches.
The structure is also disclosed as being used with all three Alternating Current (AC) phases provided through three such valve strings connected in series between two DC poles and three multilevel strings also connected in series between the two DC poles and in parallel with the valves. Thereby there are three converter modules, one for each phase, connected in series between the two poles, where each converter module comprises a director valve phase leg in parallel with a waveshaper phase leg. Furthermore, the director valve phase leg and waveshaper phase leg are both divided into an upper phase arm and a lower phase arm. Each phase arm is thus made up of half a phase leg. In this case a full-bridge cell in a waveshaper phase arm is used as a commutation cell for a corresponding director valve.
WO 2016/037666 also describes a structure with stacked converter modules.
When stacking converter modules between two DC poles in the above described way a waveshaper phase arm is typically used in a half-period of the waveshape that is formed. This means that the degree with which the cells are used in the waveshaper phase leg is fairly low. It would be of interest to improve on this situation.